An example of a sequential color display system is a display system that utilizes a digital micromirror device (DMD) to produce images for display purposes. A DMD is a type of spatial light modulator that uses a large number of micromirrors arranged in an array. Each micromirror typically pivots about an axis and can reflect light from a light source either onto a display plane or away from the display plane, with the position (state) of the micromirrors in the DMD being dependent on the image being displayed. The image on the display plane is created by the micromirrors operating in conjunction with one another.
The DMD is normally a binary spatial light modulator, i.e., the light from the light source is either reflected onto or away from the display plane. To display shades of gray or color, it is necessary for the DMD-based display system to sequentially display colors and intensities of light (or simply, color sequences), with the states of the micromirrors in the DMD potentially changing along with the changing light. The human eye integrates the many images displayed by the display system over time, producing high-quality color images.
Until recently, most DMD-based display systems used high-intensity arc lamps to provide a broad spectrum light to illuminate a single DMD. This required the use of a rotating color wheel (a form of color filter that is capable of changing its color filtering characteristics over time) to generate sequences of colored light. It is then possible to create different colors and gray scales by varying the duration of time that the individual micromirrors reflect light onto the display plane. Since the color wheel is a physical entity, it is not possible to change the sequence of colors or their relative durations once the color wheel has been manufactured. It is possible, however, to change the duration of all of the individual color segments in the sequence of colors by altering the speed of the rotation, shortening all durations by increasing the rate of rotation or lengthening all durations by decreasing the rate of rotation.
More recent DMD-based display systems use rapidly switching light sources, such as light sources with solid-state light elements (for example, light-emitting diodes and laser diodes), to illuminate the DMD with light of different wavelengths. The rapidly switching light sources can enable the elimination of the color wheel, which can permit the display system to change the sequence of colors and/or the relative durations of the individual color segments as needed to produce desired colors and gray shades. A typical DMD-based display system will include descriptors for a wide variety of sequences of colors stored in memory. A detailed description of a DMD-based display system utilizing a solid-state light source is provided in a co-assigned U.S. Pat. No. 5,706,061, entitled “Spatial Light Image Display System with Synchronized and Modulated Light Source,” issued Jan. 6, 1998, which U.S. patent is incorporated herein by reference.
With reference now to FIG. 1a, there is shown a diagram illustrating a high-level view of a portion of a prior art DMD-based display system 100. The display system 100 includes a DMD 105 that can be illuminated by a light source 110, which can be an electric arc lamp. Light produced by the light source 110 passes through a color wheel 115, which is rotated by a motor 117. The motor 117 can be controlled by a controller 120, which can also control the operation of the DMD 105 as well as the light source 110. A memory 125, coupled to the controller 120, can provide the image data for use in setting the states of the micromirrors in the DMD 105. Additionally, the memory 125 can store color sequence information, used to generate desired gray scales, shades of colors, and so forth.
With reference now to FIG. 1b, there is shown a diagram illustrating exemplary sequences of colors as generated by the display system 100. A first sequence of colors 155 includes two color cycles, with a first color cycle 160 including a duration of a first color “C1” 161, a duration of a second color “C2” 162, and a duration of a third color “C3” 163. A second color cycle 165 can also include the three colors C1, C2, and C3. Although shown as occurring in the same order as in the first color cycle 160, depending on the design of the color wheel 115, the ordering of the colors, the number of colors, or even the colors themselves in the second color cycle 165 may be different. The first color cycle 160 occurs within a first cycle time 170 and the second color cycle 165 occurs within a second cycle time 171. The combined duration of the first cycle time 170 and the second cycle time 171 can define a frame time 175, which can be the time allowed to display a single image (or a frame of an image, depending upon the nature of the display system). Again, depending on the design of a display system, a frame time can comprise a single color cycle time or multiple color cycle times.
Since the sequence of colors in the display system 100 is generated by the color wheel 115, the sequence of colors does not change over time. A second sequence of colors 180 is identical to the first sequence of colors 155. Although the sequence of colors can be identical, depending on the control of the micromirrors in the light modulator 105, the color sequences resulting from the sequence of colors can be significantly different. For example, in the first sequence of colors 155, all of traces of the color C1 can be eliminated from display on the display plane if the micromirrors in the light modulator 105 are transitioned to an off state while the color C1 is being produced, while in the second sequence of colors 180, some of the color C1 can be displayed on the display plane.
With reference now to FIG. 2a, there is shown a diagram illustrating a high-level view of a portion of a prior art DMD-based display system 200, wherein the display system 200 utilizes a rapidly switching light source 210 to illuminate a DMD 205. Rapidly switching light sources typically use solid-state light sources, such as LEDs or laser diodes, and can quickly switch on and off. The rapidly switching light source 210 can use multiple individual light elements (not shown) with the different light elements producing light at different wavelengths. This can enable the elimination of a color filter (the color wheel 115 (FIG. 1a)). A controller 220 can provide lighting control instructions to a light driver circuit 230, which can drive the rapidly switching light source 210. The controller 220 can also control the operation of the DMD 205. A memory 225, coupled to the controller 220, can provide the image data for use in setting the states of the micromirrors in the DMD 205. Additionally, the memory 225 can store color sequence information, used to generate desired gray scales, shades of colors, and so forth.
With reference now to FIG. 2b, there is shown a diagram illustrating exemplary sequences of colors as generated by the display system 200. A first sequence of colors 255 includes two color cycles, with a first color cycle 260 including a duration of a first color “C” 261, a duration of a second color “C2” 262, and a duration of a third color “C3” 263. A second color cycle 265 can also include the three colors C1, C2, and C3. Although shown as occurring in the same order as in the first color cycle 260, depending on the display system 200, the ordering of the colors, the number of colors, or even the colors themselves in the second color cycle 265 may be different. The first color cycle 260 occurs within a first cycle time 270 and the second color cycle 265 occurs within a second cycle time 271. The combined duration of the first cycle time 270 and the second cycle time 271 can define a frame time 275, which can be the time allowed to display a single image (or a frame of an image, depending upon the nature of the display system). Again, depending on the design of a display system, a frame time can comprise a single color cycle time or multiple color cycle times.
Since the sequence of colors in the display system 200 is generated by the controller 220, the sequence of colors can change over time. A second sequence of colors 280 can be different from the first sequence of colors 255, with the duration of the individual colors being different. This can be readily accomplished by controlling the rapidly switching light source 210 with color sequence information stored in the memory 225. A diagram shown in FIG. 2c illustrates specific color sequence descriptions of the sequences of colors shown in FIG. 2b, with each description of a color comprising a color name, for example “C1,” “C2,” “C3,” and so forth, and a duration time (display duration), for example 14, 14, 12, and so on, units of time.
One disadvantage of the prior art is that although the use of rapidly switching light sources can enable changes in color sequences, it has been necessary to store the color sequence information in the memory of the display system. Given a potentially large number of color sequences, the memory storage requirements (and associated costs) can be significant. Additionally, the light output of solid state light sources can change over time and as they age, therefore, extra color sequences are needed to provide compensation for changes in light output. This can further increase the memory storage requirements.